How do I stop Blender 2.8 from quitting when I close a project?
You cannot stop Blender from quitting when you close a project
I'm afraid, it does not work the way you wish it to work. Blender cannot be opened without some file loaded and it can open only one file at a time. If the file is closed, Blender is closed as well. Even when you start Blender for the first time it loads a file - that is the default scene. You can open multiple instances of Blender with different files. Then if you close one, all the other open instances will remain open. This is just how it works.
You can have multiple scenes in a file and they can have completely different settings - they can even use different render engines for example, only one scene in an open file can be active at a time, but you can switch between them without closing the file or the instance of Blender.